Doc: Improve Clang for QDoc documentation

Break page into sections to improve readability and content navigation.
Remove "debug build" which is actually wrong.
Add example of setting path to llvm-config on Windows.
Fix bad example of setting environment variable.

Task-number: QTBUG-67189
Change-Id: Ib5404533e8cecdef8ac042fc32c4d993e8d770e4
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>